Re: [TUHS] v6 RK05 bootloader question

On 12/30/15 1:14 AM, Wolfgang Helbig wrote: Wolfgang, Nothing is wrong with SimH's handling of the machine code programs that I know of. It is when you try to modify the device registers directly using deposit to effect the boot loading that the simulator doesn't operate as would a real PDP-11. For example, Norman Wilson's example: 1. Halt/Enable to Halt 2. System reset (also sends a reset to the UNIBUS) 3. Load address 777404 4. Deposit 5. (watch lights blink for a second or so) 5. Load address 0 6. Halt/Enable to Enable 7. Continue This doesn't work in SimH. I asked Mark Pizzolato about why, and he suggested the following: The reason is that on real hardware, when an I/O activity is initiated via some register probing, the device will then perform the commanded activity in parallel to the simulator's execution of instructions. A device driver will either wait for an interrupt to know when to proceed or it will read some device status register periodically or in a tight loop to determine completion. With hand entered boot code, the goal is to minimize typing of boot code and since operations will complete (from a human perspective) as soon as the user is done typing, instructions which wait for I/O completion are not provided as part of the hand typed boot code. Simh devices don't actually operate in parallel with the CPU. The concept of parallel operation is simulated by the devices performing their activities in between the execution of simulated instructions. The simh framework has the ability to allow a device simulation to schedule its activation in between some future number of instructions executed. This allows programs in the simulated system to see that some time has elapsed from when a device operation is initiated to when it completes. A hand entered bootstrap program without any loops to wait for completion status generally won't execute enough instructions to allow the desired operation to actually complete Which makes sense, but doesn't sound like a brick wall. Yes, the console method leverages side effects, granted, but the console method worked on a real PDP, just not on the simulator. It will with some modifications... To be honest, I really liked the idea of making Norman's method work in SimH because it's fun and feels historic, and if I can provide other similar examples along with some supporting text, the modification can probably be made. Then I can write another blog entry for posterity that's a little more fun and a little less technical, well that depends on the reader's perspective, but certainly light on technical details :). Thanks, Will